Steel that is highly resistant to corrosion and erosion is required in various industries, a typical example being the production of phosphoric acid by the wet process where some of the moving parts used during digestion of the rock phosphate with sulphuric acid, such as impellers and pumps, have to withstand both corrosion and erosion. This is in particular true for phosphate ores originating from Israel, Jordan, Syria, Spanish Sahara and Mexico and to a somewhat lesser extent for phosphate ore from North Carolina, Kola, Morocco, Tunisia and Togo. The corrosive and erosive conditions encountered during the digestion of these phosphate rocks with sulphuric acid are due to relative high fluoride concentration which may vary from a few hundredths to more than a tenth of a percent; the presence of varying amounts of very hard silicous material, both natural and such that is added to suppress the effect of the free fluoride content; severe cavitation enhanced by foam and gas formation during the dissolution in particular where the ore is not calcined prior to digestion; and an often reducing or at least non-oxidizing medium.
Austenitic steels, which are characterized by a so-called face centered cubic crystal lattice structure in which Fe atoms are located in the centers of the six faces of a cube, are known to have a high corrosion resistance. However, all known austenitic steels are relatively soft having Brinell hardness of 140-180. Consequently their erosion resistance is low and they are unsuitable for making equipment such as pumps and impellers for use in the processing of highly erosive liquid media such as solutions, slurries and suspensions of the kind encountered, for example, in the phosphoric acid industry.
On the other hand, hard stainless steels are known such as, for example, the one known under the designation CD-4 and many others whose Brinell hardness is within the range of 240-310. These known hard steels are, however, not austenitic and they consequently do not have the necessary corrosion resistance required for equipment for use with highly corrosive liquid media. There are also known some special steels such as Hastelloy C (Trademark) which have a fairly good resistance to corrosion but insufficient resistance to erosion, the Brinell hardness of Hastelloy C, for example, being only about 180.
Accordingly, it has hitherto not been possible to make long lasting equipment for use in the processing of highly corrosive and erosive liquid media such as encountered, for example, in the phosphoric acid industry and it is a long standing experience that pumps and impellers used, for example, in the digestion of various types of rock phosphate with sulfuric acid have to be replaced frequently, e.g. every two or three months. There has, accordingly, existed a long-felt need for a new type of steel alloys that will combine the corrosion resistance of austenitic steel with the erosion resistance of hard, non-austenitic stainless steel. However, to date such steel alloys have been unknown.
It is thus the object of the present invention to provide new austenitic and stainless steel alloys of high corrosion and erosion resistance.